54 Illinois State Lahoratorij of Natural History^ 



they lie, by very thin mesenteries. They are not closely bound 

 to the intestine, as sometimes represented in figures of other 

 earthworms, but give abundant space within for the disten- 

 sion of the intestine with food, and are therefore not them- 

 selves liable to be disturbed by the operations of digestion. 

 Posterior to somite 9, the divisions of the dorsal vessel are not 

 widely separated as they are in somites 7, 8, and 9; but the 

 double character persists, and cross sections show that there 

 are two completely separate tubes, at least at the middle of the 

 somites, throughout the remainder of the body. Possibly in 

 some cases the tubes do not unite to pass through the septa, 

 since the channel between them may reach the septum and 

 seem to continue the division through it. The dorsal vessel 

 reaches its maximum diameter in somite 14. Anterior to 

 this somite the vessel gradually decreases in size; posterior to 

 it the vessel is for some distance about equal in size to the 

 anterior division of the intestine, which it overlies and con- 

 ceals. In somites 14-19 there is a sudden increase in the size 

 of the vessel. Posterior to somite 19 the vessel is a little 

 smaller, and continues quite uniform in diameter (with a very 

 gradual decrease in size) to its termination at the posterior end 

 of the body. In somite 13 the walls of the vessel show a few 

 chloragogue cells when examined with a hand lens. Anterior 

 to this somite the vessel is devoid of this gland. Posterior to 

 somite 13 the vessel is thickly coated with the cells. 



No free lateral branches are given off from the dorsal ves- 

 sel in somite 13, but in all the somites following, two slender, 

 contorted lateral branches pass off, one on each side, just before 

 the posterior septum, and, like the aortas, are bound to the dis- 

 sepiment by a delicate mesentery. The pair in somite 14 

 reach the body-wall between the outer and inner pairs of seta% 

 and without branches pass into the integumeut. Those in 

 succeeding somites divide into several branches just before 

 reaching the l)ody-wall, some of which doubtless collect the 

 blood from the segmental organs and other structures, but 

 most of them seem to emerge from the integument. In living 

 worms the branches in adjacent somites may be seen to anas- 

 tomose with each other and to ramify extensively in the body- 

 wall. A vessel of unusual size collects blood from the clitellus 



